Various corrosion-resistant coatings for ferrous surfaces have been proposed in the prior art, and several such compositions are commercially available which are purported to prevent oxidation of said surfaces.
A first such commercial coating is a water-based resin emulsion containing a synthetic latex resin, an organic acid and propylene glycol in water applied as a nominal 2.5 mil dry film. A second such material is a low-viscosity chelating polymer apparently disclosed in U.S. Pat. No. 3,753,924. This material contains tannins, a chelation catalyst such as oxalic acid, and a cross-linking agent, such as formaldehyde. A third such commercial coating material apparently is disclosed in U.S. Pat. No. 4,086,182. According to the patent disclosure, this coating contains a synthetic binding agent which is a polymeric esterification product of an aromatic oxycarboxylic acid having phenolic hydroxyl groups, such as an ester of gallic acid or tannin with an acid anhydride.
Other conventional, commercially available corrosion-resistant coatings for ferrous surfaces include (1) oil-based red lead primers, (2) zinc chromate-based alkyd primers, (3) inorganic zinc-rich primers, (4) organic zinc-rich primers, (5) aluminized epoxy mastics, and (6) coal tar mastics.
Each of these commercially-available products, including the three above set forth in detail, serves essentially as a primer or an undercoat to which a later paint layer is applied, and these products do prevent rusting of an underlying ferrous surface so long as the coating remains in place and is undisturbed. However, once the coating is cut or chipped and the underlying ferrous surface is exposed, the underlying exposed ferrous surface rusts in a spreading pattern which undercuts the coating, so that rusting proceeds beneath the coating and the coating soon flakes and falls off the ferrous surface. This phenomenon of "undercutting" of the coating means that the coating cannot be utilized effectively to protect a ferrous surface under many different environments where the coating is subject to impact or abrasion, for example, such coatings are not useful as an undercoating for automobiles where the coating is subject to impact from stones or gravel or other foreign objects on a roadway.
The present invention now proposes a new and improved corrosion-resistant coating which not only protects the ferrous surface while the coating remains in place, but also prevents undercutting of the coating when the coating is cut, chipped, or otherwise removed from the ferrous surface. There is some evidence that the coating will also prevent oxidation of that portion of the substrate which is exposed when the coating is cut down to the metal.
While the mechanism of protection by the coating of the present invention is not fully understood at the present time, it has been found that the active ingredient or "convertant" of the corrosion-resistant coating of the present invention is attached to the ferrous surface to prevent further oxidation of the surface. In one specie of convertant, for example where the convertant is morpholine, the surface attachment is by physical or chemical adsorption to the surface. In another specie of the present invention, for example where the convertant is trimethyl phosphite, the attachment is by chemical reaction with the ferrous surface. Further, at least in the case of morpholine, the diffusion of the convertant through the coating matrix ensures the presence of adequate morpholine at the ferrous surface, and the morpholine may migrate into a narrow crack or cut in the coating to inhibit rusting of the substrate.
Thus, the convertant of the present invention prevents oxidation of a ferrous surface or prevents further oxidation of a rusted iron surface in a manner which is quite different from the manner in which the prior art compositions function.
The convertant of the present invention is preferably applied to the ferrous surface as a part of a coating composition which includes the convertant itself, a matrix ingredient for retaining the convertant on the surface, and a solvent in which both the matrix ingredient and the convertant are dissolved so that the coating can be applied, preferably by spraying. Where the coating is porous, such as an asphaltic coating, the coating composition also preferably includes a film-forming ingredient which serves to seal the matrix, so that a water-soluble convertant will not be dissolved or leached from the composition by water, and the film former also serves to improve the physical characteristics of the matrix.
For some uses of the coatings of the type hereinbefore set forth, the coating may also include an elastomer or rubber, such as polyisobutylene or a block copolymer of styrene and butadiene, to impart desirable low temperature flexing properties to the coating. Various fillers, such as finely-divided calcium carbonate, or various bulking agents, such as a solvent-swellable kaolin clay or fumed silica, can be added to the composition.